This invention provides a select fire mechanism for an M249 automatic rifle that permits the rifle to be operated to be either fully automatic or semi-automatic. The trigger assembly of this invention replaces what would be the typical trigger assembly of the M249, which effects fully automatic fire. The replacement of the standard M249 trigger assembly with the trigger assembly of this invention permits the rifle to be operated to be either fully automatic or semi-automatic according a selection made by the shooter.
In the M249 fully automatic rifle, the sear, which is part of the trigger assembly, holds back a spring-loaded operation rod assembly (herein “op rod assembly”), which is retained in the receiver, and pulling on the trigger pulls the sear out of engagement with the op rod assembly, thus permitting the op rod assembly to move under the influence of the spring acting upon the op rod assembly. Once released, the op rod moves forward and cycles internal striking mechanisms that cause a cartridge to be fired. Thereafter, expanding gases from the ignition of powder in the cartridge furnish the energy for the continued fully automatic operation or cycling of the rifle.
Immediately after firing, as the bullet traverses the barrel and passes an internal gas port prior to exiting from the muzzle, the live gases expand through appropriate ports to force the op rod assembly back against the bias of the spring that forces the op rod assembly toward the cartridge chamber. This loads the spring, and, once the pressure of the expanding gases dissipates, the spring again forces the op rod assembly forward to fire another round. So long as the trigger remains pulled, the sear remains out of the path of the op rod assembly, and the rifle continues to fire until the ammunition is exhausted. If the trigger is released, the sear moves back into the path of the op rod assembly, stopping the same and holding it in a spring-loaded position from which it can fire another round upon a subsequent squeezing of the trigger.
Because fully automatic fire is not always needed and not always desired, there is a need in the art for a rifle that includes a select fire mechanism permitting the rifle to be selectively operated as a fully automatic firearm or a semi automatic firearm.